JP-A-2009-183086 (patent literature 1) exists as a background technique for the present technical field. In this publication, there is disclosed a technique regarding a power supply system which provides an overall management of the ownership of a plurality of storage batteries when electric power is supplied from a PHEV or the like to an electric power system while eliminating any arbitrary decision of the owner of the storage batteries such that electric power is supplied stably, thus securing reliability. In this publication, there is disclosed a technique which, when electric discharging is done from a plurality of storage batteries (storage batteries installed in an electric vehicle) to an electric power system, stabilizes the supplied electric power by identifying the storage batteries installed in the electric vehicle according to excess or deficiency of electric power and giving an instruction to stop the supply of electric power to the electric power system by means of electric power matching means.